A couple of weeks ago, I had to have an emergency laser operation on my remaining half-eye and was slightly worried when the surgeon said "I think I should tell you, Mr. White, that this intervention is not entirely pain-free."

She was right. It was not pain-free in the way that having six-inch nails driven into your head by a demented Viking is not pain-free.

Her words set me thinking (not at the time, though, as my mind was occupied with trying to keep still and not scream) about how and why we actually use these double negation constructions.

Here are some examples:

The strategy is not entirely without risk

The expedition was not risk-free

His actions were not without consequence

He is not an unpleasant person

The atmosphere was not unwelcoming

The atmosphere was not without tension

In some cases, it seems as though we are trying to play down the positive aspect. ("Not pain-free sounds less painful than "painful" - or indeed "scrotum-curlingly painful" and "not unwelcoming" does not exactly suggest "effusively welcoming".)

In other cases, however, it seems that we are doing precisely the opposite, namely heightening the positive statement by using a device that usually weakens it (an atmosphere that is "not without tension" could well be intended and understood to mean an atmosphere that was positively fractious).

I remember that when I was learning some rudimentary German, my German teacher expressed her puzzlement at the English expression "not bad", "not good" and the like as they didn't occur in German. (Neither did poached eggs until I taught the hotel chef).

I explained the logic that it allowed for you to have determined that something was ok or better but that you didn't have to come to a conclusion on quite how ok to good it actually was. Further it allowed for you to escalate the answer from ok to good. As in: not bad, not bad at all.

My next report had a nicht schlecht.

In the same way the phrases you use have the benefit of the speaker avoiding having to make a considered judgement.